As a result of the strict enforcement of controls on harmful emissions from the internal combustion engines of vehicles, particularly of automobiles, a catalytic converter has been adopted as one method of reducing these emissions by converting them into less noxious substance due to their reaction when brought into contact with a catalyst. Various types of catalytic converters are available, such as the downflow type and radial type, but the basic principle lies in placing a catalyst in a casing and purifying the exhaust gas by passing it through the catalyst. The vessel to hold this converter has to be heat-resistant and endurable. The inner casing where the exhaust gas is separated from the catalyst is supposed to be the most vulnerable part of the catalytic converter vessel, because it is exposed to exhaust gas at a temperature as high as about 1000.degree. C., as well as to the weight of the catalyst particles and the exhaust gas pressure. To resist the heat and maintain rigidity under high temperature, it is fabricated of a metal plate which is strong enough to withstand high temperature.
Meanwhile the inner casing is bored with numerous holes so tiny as to prevent the catalyst particles from dropping out of the converter and prevent an increase in the exhaust resistance. Since the catalyst particles are usually in the range of 3-4 mm for reasons of productivity, effective area and activity, the tiny holes in the inner casing have to be less than 2.5 mm when measured in the direction of their minor axis. Thus the inner casing is the least productive part in the catalytic converter vessel.
The tiny holes formed in the inner casing are usually punched holes (round or oval) or louver holes on account of the relationship between diameter and plate thickness. In the case of punched holes, the diameter is so small that the punch and die are often broken and many manhours are required to maintain the tools in good condition. In the case of louver holes, the tools can last a long time but the root of a notch in the louver holes is likely to crack under thermal stress repeatedly acting on the inner casing.
The louver holes are formed by shearing without producing any chips from the plate. As illustrated in the oblique view of FIG. 1, only the cut opening is visible from the front, but from the side holes are visible. In said louver holes, stress concentration is likely to develop at the four corners of the sheared part and therefore, when a heavy thermal stress acts repeatedly, a crack is likely to develop from said four corners toward the area between the holes. Thus the durability of a plate formed with louver holes depends on the area between these holes; the wider said area, the less likely to develop a crack and the higher the rigidity; but the exhaust resistance rises.